STUDY OF ROB WATER LEVEL RISE IN COASTAL WATERS SEMARANG BAY, JAWA TENGAH PROVINCE

• Main Author: Melya Patrice Sihombing, Tera
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/68391
• Institution: Institut Teknologi Bandung

ROB is the water level that is influenced by hydrodynamic phenomena that occur in the sea. ROB occurs when the tide rises, with the influence of wave run-up, river estuary backwater and longshore currents that increase the height of ROB flooding significantly. The impact of ROB causes damage to infrastructure, changes the physical environment and decreases environmental quality as well as social and economic losses. Previous studies on ROB water level rise gave different results due to different assumptions about the components that affect ROB and there is no definite equation to determine ROB water level. This research is based on finding out what phenomena affect the ROB water level rise and there is no definite equation or provision for measuring the ROB water level. The purpose of this study is that the disclosure of ROB is expected to be able to determine the hydrodynamic phenomena that affect the ROB water level rise and to calibrate the ROB equation based on previous events. The purpose of this study was to identify the components of coastal hydrodynamics that affect ROB, formulate equations of ROB dynamics in Semarang Bay and calibrate ROB equations against previous ROB events. The rise in ROB water level in Semarang Bay is influenced by tides, wave runup, longshore current waves and backwater of the river estuary. The tides in Semarang Bay have a mixed diurnal type, which means that there is one high tide and one low tide in one day, with a formzhal value of 1.6 and a tidal period of 12 hours 54 minutes. The tidal forecast on May 23, 2022 is that the tide is 0.21 m high and the tide is -0.22 m deep. Semarang wind data is dominated by moving from the north with a percentage of 34.43% with an average wind speed of 8.34 knots or 4.3 m/s and an effective fetch length of 179.16 km. Wind-generated waves with significant wave height and period are 1.38 m and 4.41 s. The height (H_s) and depth (T_s) of significant waves were analyzed based on the bathymetry depth of Semarang Bay. Significant waves are converted to equivalent waves to account for waves that have undergone refraction, diffraction. The equivalent waves will be used in the analysis of breaking waves, wave runups and longshore current waves. Equivalent wave height (H_o^') 1.45 m with a period (T) of 4.41 s and a coefficient of refraction (K_r) 0.994. Breaking waves are calculated based on the height and period of the equivalent wave and are affected by shallowing which causes a decrease in breaking wave height. The breaking wave height (H_b) 1.5 m and depth (d_b) 1.77 m occurred with the breaking wave zone (L_b) 92.01 m from the coast. The longshore current velocity was analyzed based on the height and angle of the breaking wave formation, which was 0.35 m/s and the water area was 4.99 km2. Longshore current and runup waves arise due to breaking waves. Longshore current and run-up waves are 1.56 m and 0.13 m respectively. Backwater occurs when the tide at the mouth of the river exceeds the surface of the river water so that the river flow turns in from the sea to the river which will affect the ROB water level. The total river discharge that empties into the Semarang Bay is 872.69 m3/s and the water area is 10.5 km2. The backwater height of the river estuary is as high as 0.48 cm during high tide. ROB water level was analyzed based on tidal water level, longshore current wave height, river estuary backwater height and wave runup height. Analysis of ROB water level rise based on the equation obtained ROB water level 2.38 m and calibrated by recording ROB height on 23 May 2022 as high as 2.1 m.